1. Field of the Invention
This invention relates to a process and apparatus for reducing macrosegregation in the casting of a metal alloy ingot employing at least one substantially static magnetic field that forms the basis of an improved ingot having a fine, equiaxed grain structure and a reduced porosity.
2. Brief Description of the Prior Art
Controlling segregation in metal alloy castings, such as for example aluminum alloy ingots, to maintain a desired uniform concentration of alloying elements throughout the ingot is of particular importance in the production of high quality metal alloy ingots. Macrosegregation is a term which is used to describe segregation on a scale which is comparable to the dimensions of the ingot. It is distinct from microsegregation, which is on the scale of the spacing between the dendrite branches.
It is well known by those skilled in the art that large ingots of metal alloys usually exhibit macrosegregation which depletes the central region of the ingot of alloying ingredients. Since the alloying ingredients increase strength, this depletion results in weakened metal in the center of the ingot.
Various processes and apparatus for reducing segregation in metal alloy castings have been known, and various processes and apparatus have been used for controlling grain structure. However, none teach or suggest the improved results of the process and apparatus of the present invention.
U.S. Pat. No. 2,861,302 discloses an apparatus for the continuous casting of molten alloys, such as aluminum alloys, wherein the partially solidified material in the mold is subjected to an alternating magnetic field to cause a stirring in the molten metal. This patent states that the stirring equalizes the temperature in the casting and provides a desired structural texture.
U.S. Pat. No. 3,842,895 discloses an apparatus for reducing microsegregation and macrosegregation in metal alloy castings. It states that the apparatus reduces such segregations in continuous metal alloy castings by withdrawing heat from one region of the liquid metal in the mold to effect solidification and simultaneously adding heat to the liquid metal in a controlled manner for reducing the width of the liquid-solid mushy zone that exists between the liquidus and solidus isotherms. It states that the liquid metal alloy introduced into the mold is superheated and convection in the liquid melt within the mold is retarded by employing a transverse magnetic field.
U.S. Pat. No. 3,911,997 discloses an apparatus for metal casting for preventing microsegregation and macrosegregation at the center of a continuously cast ingot. It employs a superconducting solenoid magnet within an insulated vessel disposed in the vicinity of one side of a mold for setting up a magneto-static field in the liquid metal within the mold.
U.S. Pat. No. 4,723,591 discloses an apparatus for regulating the level of the line of contact of the free surface of a metal with a mold used in vertical casting of aluminum alloys. It discloses that the mold is surrounded by at least one annular coil in which at least one alternating electrical current is passed.
U.S. Pat. No. 4,933,005 discloses an induction stirring method including electromagnetically inducing stirring of molten metal for inducing turbulence in the molten metal and then applying a static magnetic field to minimize the turbulence induced by the electromagnetic stirring.
U.S. Pat. No. 4,709,747 discloses a casting process for aluminum alloys that involves weakening the flow currents within the liquid pool of molten metal by mechanically increasing the internal friction of the liquid pool of molten metal. It discloses an apparatus that includes a mechanical damper consisting of two or more parallel plates or concentric rings for reducing turbulence within the pool.
U.S. Pat. No. 4,530,404 and Reissue U.S. Pat. No. Re. 32,529 disclose a process for the electromagnetic casting of metals and alloys including using simultaneously a stationary electromagnetic field and a variable electromagnetic field for producing radial vibrations within the metal and for limiting the mixing effect.
U.S. Pat. No. 4,523,628 discloses a process for casting metals and continuous casting of aluminum alloys including simultaneously applying a stationary magnetic field and a variable magnetic field for generating radial vibrations in the metal.
Methods and apparatus for electromagnetic casting of metal and alloy ingots having portions of small radius of curvature are disclosed in U.S. Pat. Nos. 4,321,959 and 4,458,744. These patents state that the apparatuses include a modified shield or screening means for reducing the electromagnetic field intensity at the corners of the forming ingot by increasing local screening of the field at the corners and for reducing the containment force at the outer peripheral surface of the molten material, respectively. They disclose a modified inductor excited by an alternating current.
U.S.S.R. Patent No. 187,255 discloses ingot casting employing inner and outer electrodes positioned in the molten metal of an ingot as it forms in the mold. It states that a potential difference supplied to the inner and outer electrodes sets up a permanent radial field between them while the current passing along the central electrode sets up a permanent azimuthal field. The azimuthal field cooperates with the radial field to set up volumetric forces in a metal enclosed between the electrodes.
U.S. Pat. No. 2,944,309 discloses a continuous casting mold for casting metal alloys having a water-cooled jacket and electrical means that surrounds the body of the continuous casting mold for forming an exteriorly applied rotating magnetic field.
U.S. Pat. No. 1,721,357 discloses a process for treating metallic bodies by magnetic force to render the metallic bodies heat resistant. It states that the process prevents a change in the form of the metallic body when it is subjected to high temperatures.
Japanese Patent No. 58,163,566 discloses an iron-chromium-cobalt type alloy that is prepared by melting the alloy and pouring it into a mold placed between electromagnets producing a magnetic field. It states that the melt is solidified in the mold in a magnetic field wherein convection of the melt is prevented. The solidified alloy is kept at a temperature of 550 to 700 degrees Centigrade before aging treatment is carried out on the ingot.
Sahu, M. D., et al., "Effects of electromagnetic fields on solidification of some aluminum alloys", British Foundryman, Vol. 70, Part III, pp. 89-92 (1977), discloses that electromagnetic stirring applied externally influences the cast grain of aluminum-copper and aluminum-magnesium alloys.
Ambardar, R. et al., "Grain Coarsening by Solidification in a Steady Magnetic Field" Aluminum, 62, (6), pp. 446-448, June 1986, discloses the grain coarsening effect of a steady magnetic field on structure formation in an aluminum-4% copper alloy cast into a sodium silicate bound sand mold.
Ambardar, R., et al., "Effect of steady magnetic field on the structure of unidirectionally solidified alloy castings", Transactions of the Indian Institute of Metals, Vol. 40, No. 1, pp. 22-26, February, 1987, discloses that a steady magnetic field was used to suppress the thermal convection during unidirectional solidification of aluminum-copper castings having a completely columnar structure.
Uhlmann, D. R., et al., "The Effect of Magnetic Fields on the Structure of Metal Alloy Castings", Transaction of the Metallurgical Society of AIME, Vol. 236, pp. 527-531, April 1966, discloses a magnetic field used to damp out liquid convection during the solidification of metal alloy castings to inhibit columnar-to-equiaxed transition and the production of a structure that is columnar to the center of the casting.
Pirich, R. G., et al., "Thermal and solutal convention damping using an applied magnetic field", Washington Microgravity Sci. and Appl., NAS 8-34922, pp. 77-78, May 1985, discloses a comparison of eutectic bismuth/manganese alloy samples grown in a transverse magnetic field to samples grown without the magnetic field present. It states that samples grown at velocities below 3 cm/h (centimeters/hour) in the magnetic field show little or no deviation in eutectic morphology from those samples grown without an applied field.
In spite of these prior art disclosures, there remains a very real and substantial need for a process and apparatus for reducing undesired macrosegregation in the casting of a metal alloy ingot. Such a process and apparatus is disclosed herein and may be employed to create an improved ingot which has a refined equiaxed grain structure and a reduced pore size.